1. Technical Field of the Invention
The present invention relates to an emission of electromagnetic waves (frequency thereof is between 0.01 [THz] and 100 [THz]) (such as terahertz waves (frequency thereof is between 0.03 [THz] and 10 [THz])), for example).
2. Related Art
A broadband terahertz wave generation method by means of a difference frequency terahertz wave generation using the Cerenkov radiation has been proposed (refer to FIG. 2 of Non-Patent Document 1 and FIG. 1 of Patent Document 1). Excitation light from a light source (KTP-OPO excited by Nd-YAG laser, for example) outputting two wavelengths is emitted to a nonlinear crystal (MgO-doped LN crystal, for example) according to FIG. 2 of Non-Patent Document 1. The incident excitation light having the two wavelengths induces nonlinear polarization in the MgO-doped LN crystal. If the MgO-doped LN crystal satisfies the condition (nTHz>nopt) of the Cerenkov radiation, a spherical wave which takes the maximal value at each position corresponding to the twice of the coherence length is generated. This spherical wave has a wave front aligned in the direction of radiation angle θ satisfying the following relation, and the terahertz waves are emitted in this direction. It should be noted that nopt denotes a refractive index in the excitation light waveband of the MgO-doped LN crystal, and nTHz is a refractive index in the terahertz waveband of the MgO-doped LN crystal.cos θ=nopt/nTHz     [Non-Patent Document 1] K. Suizu, K. Koketsu, T. Shibuya, T. Tsutsui, T. Akiba, and K. Kawase, “Extremely frequency-widened terahertz wave generation using Cherenkov-type radiation,” Opt. Express 17(8), pages 6676-6681, 2009    [Patent Document 1] JP 2010-204488 A